Using high-resolution microscopy, researchers have visualized the architecture of fibers that let cells flatten and crawl along surfaces. Read more...

The fibers that give cells the ability to flatten themselves as they crawl
along a surface are laid out like a tent, researchers have discovered. One
network of fibers connects the base and top of the cells, while another
set—like a canvas stretched over tent poles—controls the shape, angles, and
placement of those fibers from above. By using super-resolution live-cell
imaging, a team of scientists was able to see the arrangement of these
molecules in the front edge of a crawling cell in three dimensions for the
first time (1).

A new model of a crawling cell’s lamella reveals how different types of fibers are laid out. Credit: JCB 10.1083/jcb.20131104

Previous methods used to visualize the flat leading edge of a crawling
cell—called the lamella—not only had low resolution but also required fixing
cells before microscopy. “It artificially flattened cells, and you lost all
the three-dimensional architecture,” said Jennifer Lippincott-Schwartz of
the National Institute of Child Health and Human Development’s Section on
Organelle Biology. “It was really the ability to do this new imaging in both
live cells and gently fixed cells that gave us the ability to see this
beautiful three-dimensional arrangement of the filaments.”

Scientists knew that the lamella contained actin arcs and dorsal and ventral
stress fibers. But they’d only been able to view the filaments from above,
looking at a single plane, so how the filaments interacted to flatten a
crawling cell wasn't clear.

Lippincott-Schwartz and her colleagues turned to structured illumination
microscopy (SIM) to look at the lamella in new detail. They not only saw how
the fibers were arranged but also could visualize the molecules of myosin-2
that apply force to the system. What they found was that the actin arcs
coupled with myosin are arranged linearly on the top of the cell. Stiff
dorsal stress fibers (the tent poles) connect the actin arcs with adhesions
on the bottom of the cell that interact with the surface below.

“The stress fiber is playing the important role of connecting an actin
contractile system in one part of the cell with an adhesion system in
another part,” Lippincott-Schwartz said.

When the actin arcs on the top of the cell contract, they tug on the tops of
the dorsal stress fibers—as the fibers tilt, the cell’s top and bottom get
closer. The newly discovered system of fibers could be relevant not only for
cells moving along a flat epithelium—a bone or the lining of an organ—but
also for cells moving through three-dimensional meshwork,
Lippincott-Schwartz hypothesized.

Her team next plans to investigate how the flattening of a cell influences the
rest of its physiology—whether water moves out of the cell when it flattens
and crawls, for example—as well as which pathways initiate crawling to begin
with.